Mass spectrometer leak detectors are well known. One type of interface in use between such a detector and atmosphere consists of a probe having a small orifice with a gas admittance of about 10.sup..sup.-3 cubic centimeters per second from the atmosphere into the vacuum region of the leak detector. Such a low admittance is necessary to prevent the entry of a large quantity of gas per unit of time which would result in damage to the delicate mass spectrometer. In operation, the probe would be moved along the wall of a chamber containing a test gas such as helium. When a leak is encountered, the test gas passes through the orifice of the probe and is detected by the mass spectrometer which is tuned to detect the presence of that particular test gas. A serious problem with such orifice probe devices is that the orifice easily becomes clogged with airborne dust or moisture. Thus, even if a leak is encountered, the test gas may not be able to pass through the clogged orifice in sufficient quantity to indicate the correct magnitude of the leak, or in the extreme case the test gas may not be able to pass through the orifice at all.
An improvement over the earlier orifice probe was disclosed in pending U.S. patent application Ser. No. 293,206, which was filed on Sept. 28, 1972, now U.S. Pat. No. 3,828,527 and assigned to Varian Associates, wherein an interface membrane of polytetrafluoroethylene was described. The polytetrafluorethylene membrane probe was not subject to the clogging problems that beset the earlier orifice type of probe.